You can open and convert .ibooks files (Apple iBooks format) to PDF document format simply by using the export function in iBooks Author on macOS / OS X. During the export process you might lose some functionality of the iBooks format, but the conversion should be doable. Books created with iBooks Author can only be viewed on an iPad (iOS) or Mac computer and you can also use the default app to export the book.
There is no simple way to achieve ibooks to pdf conversion for Windows users.Open IBOOKS file Open PDF file
Apple's free authoring tool for making great looking e-books for iPads